(CNN) -- A contract employee at the largest nuclear plant in the nation was stopped at a plant entrance Friday with a "relatively small" explosive device in his truck, officials said.

The Palo Verde Nuclear Generating Station is the largest nuclear plant in the nation.

"This would be no story at all if it weren't for where the man worked," a law enforcement official involved in the case said.

"There is no connection to terrorism. There is no threat to public safety."

The employee was on his way to work at the Palo Verde Nuclear Generating Station in Wintersburg, Arizona, at about 6 a.m. when a routine security sweep at a checkpoint found the item, according to Jim McDonald, spokesman for Arizona Public Service Company, which owns the plant.

He described the device as a "pipe bomb."

A capped pipe was found in the truck bed, said Victor Dricks, regional spokesman for the Nuclear Regulatory Commission.

The pipe had suspicious residue, but APS said initial checks failed to show any explosive material on the pipe. Watch expert give assurance that nuclear plant is safe »

The Maricopa County Sheriff's Office took the pipe and tested it.

"MCSO bomb squad tests later determined that the capped pipe was a credible explosive device," APS said in a written statement.

The man was detained and is in the custody of the sheriff's office.

The device is not something the employee would have used in his work, which deals with computers and software, sheriff's spokesman Paul Chagolla said.

He is a middle-aged out-of-state subcontractor working at the plant who resides at a Phoenix apartment complex, Chagolla said, describing the employee as cooperative.

The plant is on lockdown until a security sweep is completed, McDonald said.

"We have a large and well-trained security force that followed the procedures exactly the way they're supposed to," he said.

Dricks said officials declared an "unusual event" at the plant -- the lowest of four emergency classifications.

The device was "relatively small" and its volatility had not been determined, APS spokesman Mark Fallon said.

The pipe was "in plain view of the security officers" and the employee apparently was not attempting to hide it, he said.

The law enforcement official involved in the case said it appeared to have been tossed into the bed of the pickup "more or less at random," and could have been thrown there by someone else -- a possibility police have not ruled out.

Moreover, the device was so small that it would not have destroyed the truck if it had detonated, let alone a large facility like Palo Verde, the official said.

Based on what they know so far, authorities may not arrest the employee, the official said.

Even if someone were to smuggle a pipe bomb into a nuclear plant, the damage would not be catastrophic, said David Heyman, director of the Homeland Security Program at the Center for Strategic and International Studies.

"The Department of Homeland Security has put in place buffer programs," Heyman said.

"They've spent tens of millions [of dollars] across the country in critical infrastructure, making sure if you get through one layer, you have to get through another and another. Before you get to that reactor, you have to get through a whole lot of security," he said.

Palo Verde is the largest electrical producer of any power plant in the country -- the equivalent of nine Hoover Dams.E-mail to a friend